The introduction of the plain paper copier has resulted in a proliferation of paper copies of paper originals. A similar result is happening to electronic images, given the easy availability of digital scanners and a quick and widespread access to images throughout the Internet. It is now very difficult for the creator of an image to generate an electronic original, for which he can be assured that illegal copies will not be spread to third parties. The use of a digital watermark is a technology that aims to prevent that spread, by incorporating an identifying mark within the image that allows one to identify the source of the image in an electronic copy. It is important that the identifying mark not be disturbing or distracting to the original content of the image, while at the same time, allowing an easy identification of the source. The watermarks could be added either by the scanner or by the halftoning software.
Watermark identification may be accomplished by embedding a digital watermark in a digital or printed page that will identify the owner of rights to the image. In the past, these images have been produced and delivered in hard copy. In the future, these images will be distributed mainly in digital form. Therefore, image identification will have to work for both hard copy and digital image forms.
Watermarking can take two basic forms, visible or perceptible and invisible or imperceptible. Visible watermarks are marks such as copyright logos or symbols or logos that are imprinted into the digital or printed image to be distributed. The presence of the watermark is made clearly visible in the image in a way that makes it difficult to remove without damaging the image. The presence of the visible watermark does not harm the usefulness of the image, but it prevents the image from being used without permission. However, visible watermarks may interfere with the use of the image or with the image aesthetics. The visible watermark is also a potential target for fraud, in that it is possible for a fraudulent copier of the image to identify the location of the watermark and attempt to reproduce the image without the watermark.
Invisible watermarks are marks such as copyright symbols, logos, serial numbers, etc. that are embedded into digital or printed images in a way which is not easily discernible to the unaided eye. At a later time, the information embedded in these watermarks can be derived from the images to aid identification of the source of the image, including the owner and the individual to whom the image is sold. Such watermarks are useful for establishing ownership when ownership of an image is in dispute. They will be less likely to be useful as a deterrent to the theft of the image.
While either or both visible or invisible watermarks are desirable in an image, they represent different techniques for either preventing copying or detecting copying. It is anticipated that document producers may wish to use both kinds of protection.
The concept of conjugate screen has been used in stochastic screening for embedding information into the images. In comparing to the other data-hiding techniques such as digital watermarks, it has the advantage that it is robust to printing. In addition, the embedded information can be retrieved not only digitally, but also optically. However, it also has a few weaknesses. First, the information embedding process needs substantial computation and is difficult to be implemented in real time. Second, the size of the embedded symbol is constrained by the halftone matrix size. Third, careful registration is required in optical detection.